Method for producing hot-dip galvanized steel sheet having high strength and also being excellent in formability and galvanizing property

ABSTRACT

A hot-dip galvanized high-strength steel sheet having superior workability and galvanizability containing: 
     0.01% to 0.20% by weight of C; 
     1.0% by weight or less of Si; 
     more than 1.5% to 3.0% by weight of Mn; 
     0.10% by weight or less of P; 
     0.05% by weight or less of S; 
     0.10% by weight or less of Al; 
     0.010% by weight or less of N; 
     0.010% to 1.0% by weight in total of at least one element selected from the group consisting of Ti, Nb, and V; and 
     the balance being Fe and incidental impurities; 
     in which the steel sheet has the metal structure in which the areal rate of the ferrite phase is 50% or more, the ferrite phase has an average grain diameter of 10 μm or less, and the thickness of a band-like structure composed of the second phase satisfies the relationship Tb/T≦0.005, where Tb is the average thickness in the sheet thickness direction of the band-like structure and T is the thickness of the steel sheet, and a method for producing the same. To provide a method for producing a hot-dip galvanized high-strength steel sheet in which superior workability and high strength are obtained and moreover satisfactory galvanizability is obtained when galvanizing is performed using facilities such as a continuous galvanizing line.

TECHNICAL FIELD

The present invention relates to a method for producing hot-dipgalvanized high-strength steel sheets (including hot-dip galvannealedhigh-strength steel sheets) which are suitable for use as automotiveinner panels, outer panels, etc.

BACKGROUND ART

Recently, in view of safety, weight reduction, and improved gas mileagein automobiles, and also in view of improvement in the globalenvironment, there is a growing tendency to use hot-dip galvanizedhigh-strength steel sheets as automotive steel sheets.

In order to produce a hot-dip galvanized high-strength steel sheet, thesteel sheet must have superior galvanizability and must have the desiredstrength and workability after the steel sheet passes through a moltenzinc bath, or after the steel sheet is further subjected togalvannealing.

In general, in order to increase the strength of a steel sheet, solidsolution hardening elements, such as Mn, Si, and P, and precipitationhardening elements, such as Ti, Nb, and V, are added thereto. It isknown that when a steel sheet to which such elements have been added istreated in a continuous galvanizing line (CGL), galvanizability isdeteriorated.

Since the amounts of the alloying elements inversely affect the strengthand the galvanizability, it has been extremely difficult to produce ahot-dip galvanized high-strength steel sheet having superiorgalvanizability in the continuous galvanizing line. Additionally, sincethe hot-dip galvanized high-strength steel sheet generally has inferiorcharacteristics regarding workability, such as in elongation, it hasbeen more difficult to produce a hot-dip galvanized steel sheet havingsuperior workability.

As a conventional high-strength steel sheet having improved workability,a steel sheet with a complex structure, in which a ferrite matrixcontains a low-temperature transformed phase having martensite as aprincipal phase (also including retained austenite), is known. The steelsheet with this complex structure has non-aging properties at roomtemperature and a low yield ratio, and has superior workability andsuperior bake hardenability after working. The steel sheet with acomplex structure is produced by heating at temperatures in the ferriteand austenite (α+γ) two-phase region, followed by quenching bywater-cooling, gas-cooling, or the like.

However, when the steel sheet with a complex structure is galvanized ata temperature of approximately 500° C., or is further galvannealed,martensite distributed in the ferrite matrix is tempered, tensilestrength and elongation are decreased, and the upper yielding pointappears, resulting in an increase in yield ratio, and also yield pointelongation occurs.

Temper softening easily occurs as the amounts of alloying elements, suchas Mn and Si, are decreased. On the other hand, when the amounts of suchalloying elements are increased, hot-dip galvanizability is decreased.Ultimately, in the steel sheet with a complex structure, sincemartensite is tempered in the galvanizing process, it has been difficultto make workability and high strength, which are characteristicsthereof, compatible with each other and also to develop satisfactorygalvanizability, using the conventional techniques.

Accordingly, the applicant of the present invention has applied forother patents under International Application Nos. PCT/JP99/04385 andPCT/JP00/02547 for inventions relating to high-strength steel sheetshaving satisfactory galvanizability and methods for producing the same.

PCT/JP99/04385 is an invention relating to a high-strength steel sheetto which Mo and Cr have been added, which are significantly important inproducing a dual-phase galvanized steel sheet with a complex structurein which the matrix ferrite contains the low-temperature transformedphase having martensite as the principal phase. However, Mo and Cr arevery expensive elements and are constituents which are too costly forthe production of general-purpose, inexpensive galvanized steel sheet towhich the present invention is directed. Additionally, inPCT/JP99/04385, although Mo is added to the material containing a largeamount of Mn in order to produce a more favorably dual-phase sheet steelwith a complex structure, if Mo is added, the thickness of a band-likestructure in the steel sheet is increased. Consequently, press crackingmay occur, resulting in deterioration in workability, and in order toeliminate the band-like structure, high-temperature annealing isabsolutely necessary. Although the high-temperature heating is effectivefor galvanizability when double heating is performed, thehigh-temperature heating acts adversely when single heating isperformed, and thus it is not necessarily a condition suitable for toreconciling the two processes.

On the other hand, PCT/JP00/02547 relates to a galvanized steel sheetwith a complex structure to which 1.0% to 3.0% of Mn and 0.3% to 1.8% ofSi are added, and which contains the retained austenite phase and thetempered martensite phase which are very important in improving thestrength-elongation balance. However, in order to obtain such astructure, a primary heating-cooling process and a secondaryheating-cooling process must be combined. Additionally, in the coolingstep after heating is performed in the primary process, quenchingtreatment must be performed rapidly at a cooling rate of 10° C./s ormore, down to the Ms temperature or less, resulting in processingdifficulties. Also, in addition to a single heating-cooling processwhich is normally performed, at least one other heating-cooling processmust be performed before the CGL line.

DISCLOSURE OF INVENTION

Accordingly, in order to overcome the problems associated with theconventional techniques described above, it is an object of the presentinvention to provide a method for producing a hot-dip galvanizedhigh-strength steel sheet in which both satisfactory workability andhigh strength are provided, and moreover satisfactory galvanizability isobtained even if galvanizing is performed using facilities such as acontinuous galvanizing line.

Specifically, it is an object of the present invention to obtainsatisfactory galvanization while satisfying a TS of 590 MPa or more, anEl of 25% or more, and a value of TS×El of 15,000 MPa.% or more, asstandards for workability and high strength.

In this case, the present inventors have made every effort to carry outresearch to solve the problems described above and have discovered ahot-dip galvanized high-strength steel sheet having superior workabilityand galvanizability even if Mo and Cr are not added, and even if theretained austenite phase and the tempered martensite phase are notcontained, as well as a method for producing the same, thus achievingthe present invention.

(1) A hot-dip galvanized high-strength steel sheet having superiorworkability and galvanizability contains, in % by weight, 0.01% to 0.20%of C, 1.0% or less of Si, more than 1.5% to 3.0% of Mn, 0.10% or less ofP, 0.05% or less of S, 0.10% or less of Al, and 0.010% or less of N, andalso contains 0.010% to 1.0% in total of at least one element selectedfrom the group consisting of Ti, Nb, and V, and the balance being Fe andincidental impurities, and also has the metal structure in which theareal rate of the ferrite phase is 50% or more, the ferrite phase has anaverage grain diameter of 10 μm or less, and the thickness of aband-like structure composed of the second phase satisfies therelationship Tb/T≦0.005, where Tb is the average thickness in the sheetthickness direction of the band-like structure and T is the thickness ofthe steel sheet.

(2) A hot-dip galvanized high-strength steel sheet having superiorworkability and galvanizability contains, in % by weight, 0.01% to 0.20%of C, 1.0% or less of Si, more than 1.5% to 3.0% of Mn, 0.10% or less ofP, 0.05% or less of S, 0.10% or less of Al, and 0.010% or less of N, andalso contains 0.010% to 1.0% in total of at least one element selectedfrom the group consisting of Ti, Nb, and V, and further contains 3.0% orless in total of at least one of Cu and Ni, and the balance being Fe andincidental impurities, and also has the metal structure in which theareal rate of the ferrite phase is 50% or more, the ferrite phase has anaverage grain diameter of 10 μm or less, and the thickness of aband-like structure composed of the second phase satisfies therelationship Tb/T≦0.005, where Tb is the average thickness in the sheetthickness direction of the band-like structure and T is the thickness ofthe steel sheet.

(3) A method for producing a hot-dip galvanized high-strength steelsheet having superior workability and galvanizability includes the stepsof hot-rolling a slab having the steel composition described in (1) or(2) above, followed by coiling at 750 to 450° C.; optionally, furtherperforming cold-rolling; heating the resulting hot-rolled sheet orcold-rolled sheet to a temperature of 750° C. or more; and subjectingthe hot-rolled sheet or cold-rolled sheet to hot-dip galvanizing whilecooling from this temperature.

(4) A method for producing a hot-dip galvanized high-strength steelsheet having superior workability and galvanizability includes the stepsof hot-rolling a slab having the steel composition described in (1) or(2) above, followed by coiling at 750 to 450° C.; optionally, furtherperforming cold-rolling; heating the resulting hot-rolled sheet orcold-rolled sheet to a temperature of 750° C. or more; subjecting thehot-rolled sheet or cold-rolled sheet to hot-dip galvanizing whilecooling from this temperature; and then performing galvannealing.

(5) A method for producing a hot-dip galvanized high-strength steelsheet having superior workability and galvanizability includes the stepsof hot-rolling a slab having the steel composition described in (1) or(2) above, followed by coiling at 750 to 450° C.; optionally, furtherperforming cold-rolling; heating the resulting hot-rolled sheet orcold-rolled sheet to 750° C. or more, followed by cooling; furtherheating to a temperature of 700° C. or more; and subjecting thehot-rolled sheet or cold-rolled sheet to hot-dip galvanizing whilecooling from this temperature.

(6) A method for producing a hot-dip galvanized high-strength steelsheet having superior workability and galvanizability includes the stepsof hot-rolling a slab having the steel composition described in (1) or(2) above, followed by coiling at 750 to 450° C.; optionally, furtherperforming cold-rolling; heating the resulting hot-rolled sheet orcold-rolled sheet to 750° C. or more, followed by cooling; furtherheating to a temperature of 700° C. or more; subjecting the hot-rolledsheet or cold-rolled sheet to hot-dip galvanizing while cooling fromthis temperature; and then performing galvannealing.

That is, this may be accomplished by:

(1) positively adding at least one element selected from the groupconsisting of Ti, Nb, and V, it is possible to refine ferrite (α) grainsto 10 μm or less due to pinning of the grain boundary migration ofcarbides, such as TiC, NbC, and VC, and also it is possible to suppressthe coarsening of γ gains generated and grown in the ferrite andaustenite (α+γ) two-phase region during heating or γ gains in theaustenite (γ) single-phase region;

(2) heating, the band-like structure composed of the second phasecontaining large amounts of C and Mn is dissolved so that the thicknessof the band-like structure satisfies the relationship Tb/T≦0.005, whereTb is the average thickness in the sheet thickness direction of theband-like structure and T is the thickness of the steel sheet.

Because of synergy between (1) and (2) described above, even withoutadding Mo and Cr, and also even if the structure does not contain theretained austenite phase and the tempered martensite phase, since the γgrains before cooling are refined, the concentration of C and Mn fromthe α phase to the γ phase during cooling is increased, the γ phase iseffectively transformed into martensite, and thus a hot-dip galvanizedhigh-strength steel sheet having superior workability andgalvanizability can be produced.

In particular, in contrast to PCT/JP99/04385 and PCT/JP00/02547, sinceCr and Si, which are disadvantageous to galvanizability, are notsubstantially contained as essential elements, satisfactorygalvanizability is obtained, and since Mo is not added, the band-likestructure which is present before heating is relatively thin, and thus,even if high-temperature heating, which is disadvantageous in view ofgalvanizability, is not performed in the single CGL process, it ispossible to produce a hot-dip galvanized high-strength steel sheethaving superior workability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which shows the relationships between the heatingtemperature in a continuous galvanizing line and the tensile strength(TS), the yield strength (YS), the elongation (El), and galvanizability.

FIG. 2 is a graph which shows the relationships between the coilingtemperature and the tensile strength (TS), the yield strength (YS), theelongation (El), and galvanizability, and also shows the influence whendouble heating is performed.

BEST MODE FOR CARRYING OUT THE INVENTION

First, experimental results on which the present invention is based willbe described.

EXPERIMENT 1

A sheet bar having a thickness of 30 mm and the chemical compositionincluding 0.08% by weight of C, 0.01% by weight of Si, 1.9% by weight ofMn, 0.011% by weight of P, 0.002% by weight of S, 0.04% by weight of Al,0.0022% by weight of N, 0.02% by weight of Ti, and 0.05% by weight of Nbwas heated to 1,200° C. and rolled by a 5-pass hot rolling to produce ahot-rolled sheet with a thickness of 2.8 mm. Next, heat treatment wasperformed for 1 hour at 400° C. or 650° C., which corresponded totreatment at a coiling temperature (CT). Pickling treatment was thenperformed, followed by cold rolling to produce a cold-rolled sheet witha thickness of 1.4 mm, which was held while being heated at 700° C. to850° C. for 1 minute, and was cooled to 500° C. at a rate of 10° C./s.Galvanizing was performed, followed by holding for 40 s, andgalvannealing was performed by heating to 550° C. at a rate of 10° C./s,immediately followed by cooling to room temperature at a rate of 10°C./s. Temper rolling was then performed with a rolling reduction of1.0%.

With respect to the resulting hot-dip galvanized steel sheet, tensilecharacteristics (TS, YS, and El) were measured using JIS No. 5 testpieces for tensile testing, and galvanizability was also investigated.

In order to evaluate the galvanizability, the surfaces were visuallyinspected, using the following criteria.

◯: No non-galvanized defects (good galvanizability)

Δ: Non-galvanized defects occurred in some parts (partially goodgalvanizability)

x: Non-galvanized defects occurred over the entire surface (badgalvanizability)

The results obtained are shown in FIG. 1. As is clear from FIG. 1, whenthe coiling temperature is 650° C. and heating temperature beforegalvanizing is 750° C. or more, a TS of 590 MPa or more and an El of 25%or more can be achieved.

EXPERIMENT 2

A cold-rolled sheet with a thickness of 1.6 mm having the samecomposition as that in Experiment 1, in which the heat treatmenttemperature corresponding to CT was varied from 400° C. to 700° C., washeld at 750° C. for 1 minute (first heating), followed by cooling toroom temperature at a rate of 10° C./s, and pickling treatment was thenperformed, followed by holding at 750° C. for 1 minute (second heating)and cooling to 500° C. at a rate of 10° C./s. Galvanizing was performed,followed by holding for 40 s, and galvannealing was performed by heatingto 550° C. at a rate of 10° C./s, immediately followed by cooling toroom temperature at a rate of 10° C./s. Temper rolling was thenperformed with a rolling reduction of 1.0%.

With respect to the resulting hot-dip galvanized steel sheet, tensilecharacteristics and galvanizability were investigated in a mannersimilar to that in Experiment 1. As a result, it was found that whendouble heating treatment (first heating and second heating) is performed(indicated by ◯ in FIG. 2), as shown in FIG. 2, both tensilecharacteristics and galvanizability could be further improved incomparison with an experiment that is similar to Experiment 1 in whichsingle heating only is performed (indicated by  in FIG. 2).

As is clear from each of the experiments described above, even when thestrength of a steel sheet is increased by increasing the Mn content,galvanizability and mechanical characteristics can be improved byhigh-temperature coiling, heating at high temperatures beforegalvanizing, or double heating treatment.

The reasons for such effects are believed to be that in high-temperaturecoiling and double heating treatment, an internal oxidation layer forelements which are easily oxidized is generated just below the surfaceof the steel sheet, and thus Mn, which is disadvantageous togalvanizability, is prevented from concentrating in the surface of thesteel sheet, and a concentrated surface layer of Mn, which isdisadvantageous to galvanizability, which is generated byhigh-temperature heating, is removed by pickling treatment before thesecond heating, and that in high-temperature heating prior togalvanizing, the band structure with high concentrations of C and Mn isdissolved, which favorably affects the generation of the second phase,such as martensite.

Next, the reasons for specifying the limits in compositions andproduction conditions in the present invention will be described. (Thecompositions are shown in percent by mass.)

C: 0.01% to 0.20% by weight

Carbon is one of the important, basic elements constituting a steel, andin particular, in the present invention, carbon precipitates carbides ofTi, Nb, and V, thus increasing strength, and also improves strength viathe bainite phase and the martensite phase which are generated at lowtemperatures. If the carbon content is less than 0.01% by weight, theprecipitates, as well as the bainite phase and the martensite phase, arenot easily generated. If the carbon content exceeds 0.20% by weight,spot weldability is decreased. Therefore, the carbon content is set inthe range of 0.01% to 0.20% by weight. Additionally, the carbon contentis preferably set at 0.03% to 0.15% by weight.

Si: 1.0% by weight or less

Although silicon is an element which improves workability, such aselongation, by decreasing the amount of a solid solution of carbon inthe α phase, if the silicon content exceeds 1.0% by weight, spotweldability and galvanizability are decreased, and thus the upper limitis set at 1.0% by weight. Additionally, the silicon content ispreferably set at 0.5% by weight or less. Since it is expensive to limitthe silicon content to less than 0.005% by weight, preferably, the lowerlimit is set at 0.005% by weight.

Mn: more than 1.5% to 3.0% by weight

Manganese is one of the important components in the present invention;it is an element which suppresses the transformation in the complexstructure and stabilizes the γ phase. However, if the manganese contentis 1.5% by weight or less, the effect thereof is not exhibited, and ifthe manganese content exceeds 3.0% by weight, spot weldability andgalvanizability are significantly impaired. Therefore, manganese isadded in the range of more than 1.5% to 3.0% by weight, and preferably,in the range of 1.6% to 2.5% by weight.

P: 0.10% by weight or less

Although phosphorus is an effective element to achieve high strengthinexpensively, if the phosphorus content exceeds 0.1% by weight, spotweldability is significantly decreased, and thus the upper limit is setat 0.10% by weight. Additionally, the phosphorus content is preferablylimited to 0.05% by weight or less. Since it is expensive to limit thephosphorus content to less than 0.001% by weight, the lower limit ispreferably set at 0.001% by weight.

S: 0.05% by weight or less

Sulfur causes red shortness during hot rolling and induces cracking innuggets in the spot-welded zone, and thus the sulfur content ispreferably decreased as much as possible. Therefore, in the presentinvention, the upper limit is set at 0.05% by weight or less.Additionally, the sulfur content is more preferably limited to 0.010% byweight or less. Since it is expensive to limit the sulfur content toless than 0.0005% by weight, the lower limit is preferably set at0.0005% by weight.

Al: 0.10% by weight or less

Aluminum is an element which acts as a deoxidizing agent in the steelmaking process and which is effective in pinning N, which causes strainaging, as AlN. However, since the aluminum content exceeding 0.10% byweight results in an increase in production costs, the aluminum contentmust be limited to 0.10% by weight or less. Additionally, the aluminumcontent is preferably set at 0.050% by weight. If the aluminum contentis less than 0.005% by weight, sufficient deoxidation cannot beperformed, and thus the lower limit is preferably set at 0.005% byweight.

N: 0.010% by weight or less

Since nitrogen causes strain aging, increases the yield point (yieldratio), and causes yield elongation, the nitrogen content must belimited to 0.010% by weight or less. Additionally, the nitrogen contentis preferably set at 0.0050% by weight or less. Since it is expensive tolimit the nitrogen content to less than 0.0005% by weight, the lowerlimit is preferably set at 0.0005% by weight.

Ti, Nb, and V: 0.01% to 1.0% by weight in total

Titanium, niobium, and vanadium form carbides and are effective elementsto increase the strength of the steel, and 0.01% to 1.0% by weight of atleast one selected from the group consisting of the above elements isadded. Although the effects described above can be obtained by theaddition of 0.01% by weight or more in total of the above elements, ifthe content thereof exceeds 1.0% by weight, the cost is increased, andalso the amounts of fine precipitates excessively increase, thussuppressing recovery and recrystallization after cold rolling, and alsodecreasing ductility (elongation). Therefore, the total amount of theseelements to be added is set at 0.01% to 1.0% by weight, and preferablyat 0.010% to 0.20% by weight.

Cu and Ni: 3.0% by weight or less in total

Copper and nickel form the second phase, such as martensite, thus beingeffective elements in increasing the strength of the steel, and areadded as necessary. However, if the total content exceeds 3.0% byweight, the cost is increased, and also the yield point is decreased,which are disadvantageous when a high yield ratio is required.Therefore, the content of Cu and Ni in total is set in the range of0.010 to 3.0% by weight. Since it is expensive to limit the content ofeach element to less than 0.005% by weight, the lower limit for eachelement is preferably set at 0.005% by weight.

Ca and REM: 0.001% to 0.10% by weight

Since calcium and REM control the forms of inclusions and sulfides andimprove hole expandability, the content thereof is preferably set at0.001% by weight or more. However, if the total content exceeds 0.1% byweight, the cost is increased. Therefore, the content of Ca and REM ispreferably set in the range of 0.001% to 0.10% by weight or less, andmore preferably, the total content is set in the range of 0.002% to0.05% by weight.

Ferrite phase: 50% or more in areal rate

The present invention is directed to automotive steel sheets whichrequire high workability, and if the areal rate of the ferrite phase isless than 50%, it is difficult to maintain necessary ductility andstretch-flanging properties. Additionally, when more satisfactoryductility is required, the ferrite percentage is preferably set at 75%or more in areal rate. Examples of ferrite also include bainitic ferriteand acicular ferrite which do not contain precipitates of carbides, inaddition to so-called ferrite.

In order to observe and evaluate the ferrite phase, a steel sheet wasembedded in a resin so that the cross section of the steel sheet wasviewed, etching was performed by immersing it in a mixed solution of “anaqueous solution in which 1 g of sodium pyrosulfite was added to 100 mlof pure water” and “a solution in which 4 g of picric acid was added to100 ml of ethanol” in the ratio of 1:1, at room temperature for 120seconds, and the ferrite phase (black portion) and the second phase(white portion) were separated. The areal rate of ferrite was measuredby an image analyzer with a magnifying power of 1,000.

Average Grain Diameter of Ferrite Phase: 10 μm (0.01 mm) or less

When heating is performed by annealing to the α+γ two-phase region, ifthe ferrite grain diameter exceeds 10 μm, the size of austenite grainsgenerated from the ferrite grain boundaries increases by itself.Naturally, the large austenite grains are transformed into the secondphase, such as martensite and bainite, during cooling, and causescracking, resulting in a decrease in hole expandability. Therefore, inthe present invention, in order to refine the second phase and improvehole expandability, the ferrite grain diameter is set at 10 μm or less.

Herein, the average grain diameter is determined by the value which islarger when compared between the value measured by planimetry accordingto ASTM based on a photograph of the sectional structure and the nominalgrain diameter measured by a cutting method (for example, reported byUmemoto, et al. in “Thermal Treatment” 24 (1984) 334). Additionally, inthe present invention, it is not necessary to particularly specify thetypes of the second phase (e.g., martensite, bainite, pearlite, andcementite).

Thickness of Band-like Structure: Tb/T≦0.005

The band-like structure includes a group of second phases in whichconcentrated surface layers of C and Mn which cohere along grainboundaries mainly in the cooling process of the slab are rolled duringhot rolling or during the subsequent cold rolling and are formed like acolumn or layer in the rolling direction and in the sheet widthdirection, in a steel having large amounts of C and Mn. The reason forsetting the ratio Tb/T of the average thickness Tb of such a band-likestructure to the thickness T of the steel sheet at 0.005 or less is thatwhen a large amount of Mn is contained as in the present invention, thethickness of the band-like second phase structure containing C and Mn asprincipal ingredients is increased in the structure of the hot-rolledsheet, resulting in a difficulty in producing a high-strength steelsheet in which hard martensite is homogeneously dissolved in the ferritematrix. Consequently, in order to efficiently produce a high-strengthsteel sheet, C and Mn which are concentrated in the band-like secondphase must be dissolved, and the ratio of the average thickness Tb ofthe band-like structure and the thickness T of the sheet serves as ameasure thereof. If the relationship Tb/T≦0.005 is satisfied, goodresults can be obtained.

In order to observe and evaluate the thickness Tb of the band-likestructure, a steel sheet was embedded in a resin so that the crosssection of the steel sheet was viewed, etching was performed byimmersing it in a 3% nital solution at room temperature for 15 seconds,and 20 pieces of column-like, layered structure of the second phase weremeasured by an image analyzer with a magnifying power of 1,500 to obtainthe average thickness Tb.

Next, the production conditions in the present invention will bedescribed.

A steel slab having the composition described above is hot-rolled by aconventional method, followed by coiling at 750 to 450° C. If thecoiling temperature is less than 450° C., carbides, such as TiC and NbC,are not easily generated, resulting in a shortage in strength, and aninternal oxidation layer is not easily formed just below the surface ofthe steel sheet, thus being unable to suppress the concentration of Mnin the surface of the steel sheet. On the other hand, if coiling isperformed at a temperature exceeding 750° C., the thickness of a scaleis increased and pickling efficiency is decreased, and also variationsin material quality are increased among the tip, center, and rear end inthe longitudinal direction of the coil, and the edge section and thecenter section in the coil width direction. Additionally, the coilingtemperature is preferably set at 700 to 550° C.

The hot-rolled sheet is descaled by pickling treatment, as necessary,and as hot-rolled, or after cold-rolling is further performed, heatingis performed at 750° C. or more by a continuous galvanizing line,followed by cooling, and then galvanizing is performed while cooling.

When double heating is performed, first, heating (first heating) isperformed at 750° C. or more by a continuous annealing line or the like.Next, after cooling is performed, heating (second heating) is performedat 700° C. or more by a continuous galvanizing line, followed bycooling, and galvanizing is performed, preferably, at 420 to 600° C.,while cooling.

By heating in the temperature range of 750° C. or more (preferably, 750to 900° C.), followed by cooling, prior to galvanizing, Mn, etc.,concentrated in the band-like structure are dissolved, and the complexstructure including ferrite and martensite is efficiently formed, thusimproving workability. That is, when the Mn content is increased as inthe present invention, the band-like second phase is easily formed inthe hot-rolled sheet, and the concentration of Mn, etc., in the γ phaseis decreased, which is disadvantageous to the formation of the complexstructure. Therefore, by decreasing the thickness of the band-likestructure and by finely dissolving Mn, etc., when the temperature ismaintained at approximately 500° C. in the galvanizing process in thecontinuous galvanizing line, or further in the galvannealing process,the Mn content concentrated in the γ phase is increased, and thus themartensite phase can be properly dissolved in the ferrite matrix.

When double heating is performed, the second heating is performed at700° C. or more. The second heating is inevitably performed in thecontinuous galvanizing line. If the second heating temperature is lessthan 700° C., the surface of the steel sheet is not reduced, andgalvanizing defects easily occur. The second heating temperature ispreferably set in the range of 750 to 800° C. Additionally, when doubleheating is performed, pickling treatment is preferably performed inorder to remove the concentrated surface layer of Mn, etc., generated inthe first heating and to improve galvanizability thereafter. Thepickling treatment is performed, preferably, at 30 to 70° C., in a 1 to10% HCl solution, for approximately 3 to 10 s.

Subsequent to the heating process described above, galvanizing isperformed, and in some cases, after galvanizing is performed,galvannealing may be performed successively.

EXAMPLE 1

Continuously cast slabs with a thickness of 300 mm having the chemicalcompositions shown in Table 1 were heated to 1,200° C., and were roughlyrolled by 3-pass rolling, and then were hot-rolled by a 7-standfinishing rolling mill to form hot-rolled sheets with a thickness of 2.5mm, followed by coiling. The hot-rolled sheets were subjected topickling treatment, and as the hot-rolled sheets, or after thehot-rolled sheets were further cold-rolled to a thickness of 1.2 mm,galvanizing was performed in a process (1) including first heating in acontinuous annealing line—pickling—second heating in a continuousgalvanizing line, or a process (2) including heating in a continuousgalvanizing line—galvanizing. Furthermore, with respect to samplescollected from portions thereof, galvannealing was performed. Theproduction conditions for the above are shown in Table 2.

Additionally, as the CGL conditions after heating, the average coolingrate for the steel sheets from heating to galvanizing was set at 10°C./s, immersion in a galvanizing bath with the conditions describedbelow was performed, and then the areal weight was adjusted to 60 g/m²by gas-wiping. Next, heating was performed to 490° C., followed byholding for 20 s, and then cooling was performed to 200° C. or less atan average cooling rate of 20° C./s.

Composition: 0.15% Al—Zn

Temperature: 470° C.

Immersion time: 1 s

With the resulting steel sheets being treated as samples, mechanicalcharacteristics, galvanizability, spot weldability, etc., wereinvestigated. The results thereof are shown in Table 2.

Herein, mechanical characteristics, galvanizability, galvannealability,and spot weldability were evaluated by the methods described below.

Mechanical characteristics (investigated by tensile test and holeexpanding test)

Using No. 5 test pieces according to JIS Z 2204 collected from the steelsheets in a direction at right angles to the rolling direction, yieldstrength (YS), tensile strength (TS), elongation at break (El), andyield elongation (YEl) were measured according to JIS Z 2241.

In order to investigate stretch-flanging properties, the hole expandingrate (λ) was measured by a hole expanding test according to JFS T 1001.

Galvanizability

Good: No non-galvanized defects

Partially Good: Non-galvanized defects occurred in some parts

Bad: Many non-galvanized defects occurred

Galvannealability

Good: Completely free from galvannealing blurs

Partially Good: Galvannealing blurs slightly observed

Bad: Galvannealing blurs significantly observed

Spot Weldability

Spot welding was performed under the following welding conditions. Thatis, a welding electrode with a dome tip diameter of 6 Φ was used with anelectrode force of 3.10 kN, a welding current of 7 kA, a squeeze time of25 cyc., a setup time of 3 cyc., a welding time of 13 cyc., and aholding time of 25 cyc. A tensile load by a tensile shear test accordingto JIS Z 3136 (TSS) and a tensile load by a cross tensile test accordingto JIS Z 3137 (CTS) were applied, and the test pieces in which thetensile shear loads were 8,787 N or more corresponding to the standardtensile shear load at a sheet thickness of 1.2 mm, and in which theductility ratio (CTS/TSS) is 0.25 or more were evaluated as “superior”,and the test pieces which did not satisfy the above values wereevaluated as “inferior”.

As is clear from Tables 1 and 2, in the examples of the presentinvention, tensile properties with a TS of 590 to 690 MPa and an El of25% by weight or more are observed, and satisfactory TS×El balances with15,000 MPa.wt % or more are observed, and also there is no particularproblems with respect to galvanizability, galvannealability, and spotweldability.

EXAMPLE 2

Continuously cast slabs with a thickness of 300 mm having the chemicalcompositions shown in Table 3 were heated to 1,200° C., and were roughlyrolled by 3-pass rolling, and were then hot-rolled by a 7-standfinishing rolling mill to form hot-rolled sheets with a thickness of 3.0mm, followed by coiling at temperatures shown in Table 4. The hot-rolledsheets were subjected to pickling treatment, and as the hot-rolledsheets, or after the hot-rolled sheets were further cold-rolled to athickness of 1.2 mm, galvanizing was performed in a process (1)including first heating in a continuous annealing line—pickling—secondheating in a continuous galvanizing line, or a process (2) includingheating in a continuous galvanizing line—galvanizing. Furthermore, withrespect to samples collected from portions thereof, galvannealing wasperformed. The production conditions for the above are shown in Table 4.

Galvanizing was performed in a process (1) including first heating in acontinuous annealing line—pickling—second heating in a continuousgalvanizing line, or a process (2) including heating in a continuousgalvanizing line—galvanizing. Furthermore, with respect to some portionsthereof, galvannealing was performed. The production conditions for theabove are shown in Table 4.

With the resulting steel sheets being treated as samples, mechanicalcharacteristics, galvanizability, spot weldability, etc., were evaluatedin a similar manner. The results thereof are also shown in Table 4.

Additionally, as the CGL conditions after heating, the average coolingrate for the steel sheets from heating to galvanizing was set at 10°C./s, immersion in a galvanizing bath with the conditions describedbelow was performed, and then the areal weight was adjusted to 60 g/m²by gas-wiping. Next, heating was performed to 490° C., followed byholding for 20 s, and then cooling was performed to 200° C. or less atan average cooling rate of 20° C./s.

Composition: 0.15% Al—Zn

Temperature: 470° C.

Immersion time: 1 s

Areal weight: 60 g/m²

As a result, it has been found that, in the examples of the presentinvention, the TS×El balances are satisfactory, and although highstrength is obtained, there are no problems with respect togalvanizability, galvannealability, and spot weldability.

EXAMPLE 3

Continuously cast slabs with a thickness of 300 mm having the chemicalcompositions shown in Table 5 were heated to 1,200° C., and were roughlyrolled by 3-pass rolling, and were then hot-rolled by a 7-standfinishing rolling mill to form hot-rolled sheets with a thickness of 3.0mm, followed by coiling at temperatures shown in Table 6. After picklingtreatment was performed, the sheets were cold-rolled to a thickness of1.2 mm, and galvanizing was performed in a process including firstheating in a continuous annealing line—pickling—second heating in acontinuous galvanizing line, and then galvannealing was performed. Theproduction conditions for the above are shown in Table 6.

With the resulting steel sheets being treated as samples, mechanicalcharacteristics, galvanizability, spot weldability, etc., were evaluatedin a similar manner. The results thereof are also shown in Table 6.

Additionally, as the CGL conditions after heating, the average coolingrate for the steel sheets from heating to galvanizing was set at 10°C./s, immersion in a galvanizing bath with the conditions describedbelow was performed, and then the areal weight was adjusted to 60 g/m²by gas-wiping. Next, heating was performed to 490° C., followed byholding for 20 s, and then cooling was performed to 200° C. or less atan average cooling rate of 20° C./s.

Composition: 0.15% Al—Zn

Temperature: 470° C.

Immersion time: 1 s

Areal weight: 60 g/m²

As a result, it has been found that, in the examples of the presentinvention, the TS×El balances are satisfactory, and although highstrength is obtained, there are no problems with respect togalvanizability, galvannealability, and spot weldability.

INDUSTRIAL APPLICABILITY

As described above, in accordance with the present invention, it ispossible to provide a hot-dip galvanized high-strength steel sheet inwhich satisfactory galvanizability is obtained, the yield ratio isdecreased, the TS×El balance is satisfactory. Therefore, the presentinvention can reduce weight and improve gas mileage in automobiles, thusgreatly contributing to improvement in the global environment.

TABLE 1 Steel C Si Mn P S Al N Ti Nb V Remarks A 0.075 0.01 2.4 0.0070.003 0.05 0.0022 0.02 0.05 — Applicable steel B 0.101 0.02 2.3 0.0090.002 0.01 0.0032 0.21 0.03 — Applicable steel C 0.056 0.02 2.2 0.0120.001 0.05 0.0025 0.01 0.03 0.03 Applicable steel D 0.068 0.01 1.6 0.0110.001 0.07 0.0033 0.06 — — Applicable steel E 0.098 0.04 1.8 0.012 0.0020.06 0.0026 — 0.07 — Applicable steel F 0.051 0.01 1.7 0.012 0.001 0.040.0031 0.01 — 0.05 Applicable steel G 0.084 0.01 1.6 0.008 0.001 0.020.0026 0.06 0.02 0.03 Applicable steel H 0.064 0.02 1.5 0.009 0.002 0.030.0025 0.02 0.04 — Applicable steel I 0.039 0.02 1.6 0.005 0.003 0.040.0021 0.05 0.05 — Applicable steel J 0.163 0.03 1.6 0.016 0.002 0.050.0029 0.09 0.03 0.02 Applicable steel K 0.022 0.01 2.6 0.008 0.002 0.040.0027 0.07 0.01 — Applicable steel L 0.074 0.01 1.7 0.01 0.001 0.040.0028 — — — Comparative steel M 0.007 0.02 1.8 0.009 0.002 0.04 0.00210.025 — — Comparative steel N 0.082 0.02 0.7 0.026 0.002 0.03 0.00280.016 — — Comparative steel O 0.095 0.05 1.7 0.113 0.004 0.06 0.00320.033 — — Comparative steel

TABLE 2 First Second Areal Average heating heating rate of grain CTtemperature temperature ferrite diameter YS TS EL YEL No. Steel ° C.Cold rolling ° C. ° C. grains % μm Tb/T MPa MPa % % 1 A 640 Notperformed 800 750 80 3.5 0.003 389 595 30 0.0 2 A 680 Performed 770 — 763.1 0.004 402 631 29 0.0 3 B 640 Performed 850 720 70 2.3 0.002 396 64530 0.0 4 B 650 Performed 725 700 30 2.1 0.008 850 951 11 0.0 5 B 550Performed 840 — 76 1.9 0.002 411 653 29 0.0 6 C 530 Performed 850 800 854.2 0.002 362 595 32 0.0 7 C 400 Performed 850 775 82 3.5 0.003 396 62426 0.0 8 D 670 Performed 850 750 83 2.7 0.002 441 651 27 0.0 9 D 570 Notperformed 700 850 80 3.6 0.003 458 668 26 0.0 10 D 570 Performed 800 —78 2.8 0.004 448 631 27 0.0 11 E 620 Not performed 775 730 76 3.3 0.004432 596 28 0.0 12 E 620 Not performed 775 — 75 3.5 0.004 441 608 25 0.013 E 620 Performed 700 800 70 3.0 0.004 499 697 24 0.0 14 F 630Performed 840 750 82 3.5 0.002 388 598 30 0.0 15 F 620 Performed 800 —80 3.2 0.003 463 613 25 0.0 16 F 400 Performed 850 750 78 2.8 0.002 467633 24 0.0 17 F 500 Not performed 750 690 55 2.2 0.010 608 768 18 0.0 18G 640 Performed 840 775 80 3.6 0.003 443 634 26 0.0 19 G 640 Notperformed 850 800 82 3.8 0.003 443 624 30 0.0 20 G 640 Not performed 830750 78 3.4 0.004 440 612 25 0.0 21 H 530 Performed 840 800 85 4.1 0.002448 633 24 0.0 22 H 620 Performed 850 725 83 3.4 0.002 453 653 23 0.0 23I 700 Performed 820 750 90 8.2 0.003 403 595 30 0.0 24 I 650 Performed850 730 87 7.4 0.002 401 608 31 0.0 25 J 600 Performed 775 750 64 1.90.00S 402 630 26 &.0 26 K 620 Performed 850 750 92 9.8 0.002 432 610 300.0 27 L 650 Performed 880 730 86 11.0 0.002 489 550 28 2.2 28 M 700 Notperformed 825 700 97 15.0 0.001 305 496 33 0.8 29 N 650 Not performed850 650 92 12.0 0.001 260 470 35 1.5 30 O 700 Not performed 730 750 357.0 0.007 602 762 19 0.8 Hole TS × El expandability Spot No. YR % MPa %Galvanizability Galvannealability λ % weldability Remarks  1 65 17850Good Good 81 Superior Example of present invention  2 64 18299 PartiallyGood Partially Good 89 Superior Example of present invention  3 61 19350Good Good 90 Superior Example of present invention  4 89 10461 Bad Bad22 Superior Comparative Example  5 63 18937 Partially Good PartiallyGood 95 Superior Example of present invention  6 61 19040 Good Good 92Superior Example of present invention  7 63 16224 Bad Bad 90 SuperiorComparative Example  8 68 17577 Good Good 99 Superior Example of presentinvention  9 69 17368 Bad Bad 84 Superior Comparative Example 10 7117037 Partially Good Partially Good 95 Superior Example of presentinvention 11 72 16688 Good Good 83 Superior Example of present invention12 73 15200 Partially Good Partially Good 81 Superior Example of presentinvention 13 72 16728 Bad Bad 92 Superior Comparative Example 14 6517940 Good Good 101  Superior Example of present invention 15 76 15325Partially Good Partially Good 105  Superior Example of present invention16 74 15192 Bad Bad 110  Superior Comparative Example 17 79 13824 BadBad 41 Superior Comparative Example 18 70 16484 Good Good 92 SuperiorExample of present invention 19 71 18720 Good Good 86 Superior Exampleof present invention 20 72 15300 Good Good 93 Superior Example ofpresent invention 21 71 15192 Good Good 83 Superior Example of presentinvention 22 69 15019 Good Good 91 Superior Example of present invention23 68 17850 Good Good 112  Superior Example of present invention 24 6618848 Good Good 118  Superior Example of present invention 25 64 16380Good Good 86 Superior Example of present invention 26 71 18300 Good Good103  Superior Example of present invention 27 89 15400 Good Good 53Superior Comparative Example 28 61 16368 Good Good 72 SuperiorComparative Example 29 55 16450 Bad Bad 68 Superior Comparative Example30 79 14478 Bad Bad 37 Inferior Comparative Example

TABLE 3 Steel C Si Mn P S Al N Ti Nb V Cu Ni Ca REM Remarks P 0.097 0.011.8 0.005 0.001 0.08 0.0033 0.02 0.06 0 0.5 0.3 0 0.01 Applicable steelQ 0.075 0.02 1.9 0.008 0.001 0.04 0.0030 0.02 0.11 0 1.2 0.8 0 0Applicable steel R 0.056 0.09 2.5 0.009 0.001 0.05 0.0025 0.01 0.09 00.9 0 0.003 0 Applicable steel S 0.062 0.05 1.6 0.007 0.001 0.06 0.0033— 0.07 0.03 1.2 0.8 0 0 Applicable steel T 0.092 0.04 3.2 0.012 0.0020.06 0.0026 0.05 0.09 0.02 0 0 0.002 0 Comparative steel U 0.181 0.9 1.80.008 0.001 0.04 0.0031 0.01 0.11 0.02 0 0 0.003 0 Applicable steel V0.225 0.01 2.3 0.025 0.001 0.03 0.0028 0   0.02 0 0 0 0 0 Comparativesteel W 0.061 0.01 1.9 0.009 0.001 0.08 0.0033 0.15 0.04 0 0 0 0 0Applicable steel X 0.081 0.01 1.9 0.008 0.001 0.04 0.0030 0.02 0.11 00.5 0 0 0 Applicable steel Y 0.044 0.01 2.6 0.018 0.001 0.05 0.0025 0.010.14 0 0.5 0 0 0 Applicable steel Z 0.074 0.01 1.8 0.007 0.001 0.060.0033 — 0.07 0.03 0.5 0.3 0 0 Applicable steel

TABLE 4 First Second Areal Average heating heating rate of grain CTtemperature temperature ferrite diameter YS TS EL YEL No. Steel ° C.Cold rolling ° C. ° C. grains % μm Tb/T MPa MPa % % 31 P 550 Notperformed 850 750 80 2.1 0.003 596 812 20 0.0 32 P 550 Performed 850 75080 1.9 0.003 622 885 17 0.0 33 Q 550 Performed 850 725 78 1.8 0.003 618815 20 0.0 34 Q 350 Performed 850 750 77 1.7 0.003 713 855 15 0.0 35 R550 Performed 850 750 80 2.3 0.004 511 782 22 0.0 36 R 550 Performed 850800 81 2.1 0.003 498 803 24 0.0 37 S 550 Performed 850 750 82 2.2 0.002553 846 18 0.0 38 S 550 Performed — 730 * * 0.015 789 869  5 0.0 39 T550 Performed 850 750 65 1.6 0.005 458 668 26 0.0 40 U 550 Performed 850750 55 1.5 0.001 624 812 26 0.0 41 U 550 Not performed 850 750 58 1.70.001 604 806 28 0.0 42 V 550 Not performed 850 750 47 1.5 0.003 701 93210 0.0 43 W 550 Performed 850 750 80 2.4 0.003 489 677 24 0.0 44 W 550Performed 850 750 78 2.1 0.003 468 639 30 0.0 45 X 550 Performed 850 75082 2.7 0.003 533 723 22 0.0 46 Y 550 Performed 850 775 88 5.2 0.002 481633 26 0.0 47 Z 550 Not performed 850 750 81 2.4 0.003 499 674 24 0.0 48Z 550 Performed — 800 83 1.9 0.004 510 711 22 0.0 Hole TS × Elexpandability Spot No. YR % MPa % Galvanizability Galvannealability λ %weldability Remarks 31 73 16240 Good Good 83 Superior Example of presentinvention 32 70 15045 Good Good 87 Superior Example of present invention33 76 16300 Good Good 92 Superior Example of present invention 34 8312825 Bad Bad 93 Superior Comparative Example 35 65 17204 Good Good 102 Superior Example of present invention 36 62 19272 Good Good 105 Superior Example of present invention 37 65 15228 Good Good 86 SuperiorExample of present invention 38 91  4345 Good Good 10 SuperiorComparative Example 39 69 17368 Bad Bad 43 Inferior Comparative Example40 77 21112 Good Good 88 Superior Example of present invention 41 7522568 Good Good 85 Superior Example of present invention 42 75  9320Good Good 15 Inferior Comparative Example 43 72 16248 Good Good 88Superior Example of present invention 44 73 19170 Good Good 91 SuperiorExample of present invention 45 74 15906 Good Good 86 Superior Exampleof present invention 46 76 16458 Good Good 80 Superior Example ofpresent invention 47 74 16176 Good Good 86 Superior Example of presentinvention 48 72 15642 Good Good 89 Superior Example of presentinvention * Unable to measure due to unrecrystallization

TABLE 5 Steel C Si Mn P S Al N Ti Nb V Cu Ni Ca REM Remarks a 0.062 0.252.9 0.007 0.001 0.042 0.0074 0.128 0.066 0 0.05 0.02 0 0 Applicablesteel b 0.071 0.03 2.7 0.009 0.001 0.035 0.0026 0.023 0.09 0 0.01 0.01 00.01 Applicable steel c 0.012 0.35 2.3 0.006 0.011 0.045 0.0032 0.1050.06 0 0.02 0.02 0.003 0.002 Applicable steel d 0.122 0.05 3.3 0.0070.001 0.026 0.0024 — 0.07 0.03 0.95 0.31 0 0 Applicable steel e 0.0921.12 2.7 0.009 0.001 0.052 0.0056 0.05  0.09 0.02 0 0 0.002 0Comparative steel f 0.205 0.02 2.7 0.007 0.001 0.042 0.0029 0.08  0.080.02 0 0 0.003 0 Comparative steel g 0.195 0.01 2.3 0.113 0.001 0.0330.0028 — 0.02 0 0 0 0 0 Comparative steel h 0.084 0.03 2.8 0.011 0.0520.012 0.0029 0.15  0.04 0 0 0 0 0 Comparative steel i 0.081 0.01 3.00.015 0.001 0.041 0.0067 — — 0 0.3 0.15 0 0 Comparative steel j 0.0770.02 2.7 0.018 0.001 0.033 0.0025 0.003 0.005 0 0.5 0 0 0 Comparativesteel k 0.008 0.01 1.6 0.023 0.001 0.055 0.0033 — 0.07 0.03 0.5 0.3 0 0Comparative steel l 0.066 0.05 1.7 0.007 0.001 0.038 0.0069 0.028 0.0710 1.9 1.2 0 0 Comparative steel m 0.063 0.02 2.9 0.008 0.0o1 0.0360.0032 0.023 0.066 0 2.2 0.9 0 0.02 Comparative steel

TABLE 6 First Second Areal Average heating heating rate of grain CTtemperature temperature ferrite diameter YS TS EL YEL No. Steel ° C.Cold rolling ° C. ° C. grains % μm Tb/T MPa MPa % % 49 a 700 Performed850 750 65 1.9 0.004 596 993 17 0.0 50 a 400 Performed 850 750 66 1.80.004 602 1022 16 0.0 51 b 700 Performed 850 750 59 1.8 0.004 618 983 180.0 52 b 700 Performed 850 680 57 1.8 0.003 602 893 18 0.0 53 c 700Performed 850 750 63 2.0 0.004 511 812 19 0.0 54 d 700 Performed 850 75056 2.2 0.011 553 1020 12 0.0 55 e 700 Performed 850 750 56 2.1 0.005 458668 16 0.0 56 f 700 Performed 850 750 47 1.5 0.006 624 812 15 0.0 57 g700 Performed 850 750 63 1.5 0.003 701 932 15 0.0 58 h 700 Performed 850750 64 2.4 0.003 735 1025 12 0.0 59 i 700 Performed 850 750 52 2.5 0.004533 853 17 0.0 60 j 700 Performed 700 750 61 2.3 0.003 480 987 15 0.0 61k 700 Performed 850 750 99 18.0 0.001 322 381 38 0.0 62 l 700 Performed850 750 81 2.7 0.002 542 826 18 0.0 63 m 700 Performed 850 750 78 2.40.003 689 996 15 0.0 Hole TS × El expandability Spot No. YR % MPa %Galvanizability Galvannealability λ % weldability Remarks 49 60 16881Good Good 36 Superior Example of present invention 50 59 16352 Bad Bad33 Superior Comparative Example 51 63 17694 Good Good 42 SuperiorExample of present invention 52 67 16074 Bad Bad 45 Superior ComparativeExample 53 63 15428 Good Good 38 Superior Example of present invention54 54 12240 Good Good 24 Inferior Comparative Example 55 69 10688 BadBad 32 Inferior Comparative Example 56 77 12180 Good Good 14 InferiorComparative Example 57 75 13980 Good Good 12 Inferior ComparativeExample 58 72 12300 Good Good 10 Inferior Comparative Example 59 6214501 Good Good 42 Inferior Comparative Example 60 49 14805 Good Good 40Inferior Comparative Example 61 85 14478 Good Good 82 SuperiorComparative Example 62 66 14868 Bad Bad 35 Superior Comparative Example63 69 14940 Good Good 36 Inferior Comparative Example

What is claimed is:
 1. A method for producing a hot-dip galvanizedhigh-strength steel sheet having superior workability andgalvanizability, wherein the steel sheet has a metal structure in whichthe areal rate of a ferrite phase is 50% or more, the ferrite phase hasan average grain diameter of 10 μm or less, and the thickness of a bandstructure comprising a second phase satisfies the relationshipTb/T≦0.005, where Tb is the average thickness in the sheet thicknessdirection of the band structure and T is the thickness of the steelsheet, the method comprising the steps of: hot-rolling a slabcomprising: 0.01% to 0.20% by weight of C; 1.0% by weight or less of Si;more than 1.5% to 3.0% by weight of Mn; 0.10 by weight or less of P;0.05% by weight or less of S; 0.10% by weight or less of Al; 0.010% byweight or less of N; 0.010% to 1.0% by weight in total of at least oneelement selected from the group consisting of Ti, Nb, and V; and thebalance being Fe and incidental impurities; coiling the hot-rolled sheetat 750 to 450° C.; performing, optionally, cold-rolling; heating theresulting hot-rolled sheet or cold-rolled sheet to 750° C. or more;cooling and then heating the sheet to a temperature of 700° C. or more;and subjecting the sheet to hot-dip galvanizing during a cooling stepfrom this temperature.
 2. A method for producing a hot-dip galvanizedhigh-strength steel sheet having superior workability andgalvanizability according to claim 1, wherein the slab further comprises3.0% by weight or less in total of at least one of Cu and Ni.
 3. Amethod for producing a hot-dip galvanized high-strength steel sheethaving superior workability and galvanizability according to claim 1,wherein the slab further comprises 0.001% to 0.10% by weight or less intotal of at least one of Ca and REM.
 4. A method for producing a hot-dipgalvanized high-strength steel sheet having superior workability andgalvanizability according to claim 2, wherein the method furthercomprises the step of galvannealing the sheet.